How Gas Pumps Work

This is the part of the gas pump that has most visibly evolved over the years. Until a few decades ago, rotating wheels with numerals on them ticked off the number of gallons and price of gas as the pump operated. In the 1970s, glowing LCDs in the form of seven-segment displays began to appear. (The segments in the display could be illuminated by computer to form various numerals and occasionally letters of the alphabet.) These relatively simple user interfaces are gradually being replaced by full-fledged computer video displays, many running variations on operating systems like Microsoft Windows. These displays can offer information, display the amount of gas being sold and even run advertisements and carry on simple conversations with amused customers.

The Automatic Shut-off

When the customer removes the pump handle from its place on the side of the dispenser, this action activates a switch that starts the dispenser operation. (In some cases the switch is spring-loaded and activates automatically; in others, the customer must raise a small lever manually to begin the process.) At that point, the customer simply inserts the nozzle into the car's gas tank and pulls the lever. Stopping the flow of gas is just as simple -- the customer need only release the lever to cut off the stream of fuel.

But what if the tank fills unexpectedly to the brim and the gasoline threatens to overflow? As anyone who's ever operated a gas pump knows, the pump will switch off automatically. But how does the pump know when to stop pumping?

As the gas level in the tank rises, the distance between the dispenser nozzle and the fuel grows smaller. A small pipe called a venturi runs alongside the gas nozzle. When the end of the venturi pipe becomes submerged in the rising gas, it chokes off the air pressure that holds the nozzle handle open and shuts down the flow of gas. Unfortunately, this shutdown can sometimes happen before the tank is full as the rapidly flowing gas backs up on its way into the tank. This can cause the gas handle to spring open before pumping is complete, leaving the annoyed customer to squeeze the handle again and risk the possibility of overflow. Pausing briefly will allow the gas to continue into the tank and the pump nozzle to start pouring gas again.

For more information on fuel and fuel efficiency, take a look at the links on the next page.

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